G-protein coupled receptors (GPCRs) have many functions in human health and disease. About 30% of FDA approved drugs target these types of receptors. Chemokine receptors belong to a subfamily of GPCRs, are activated by small proteins of 8 kDa, and are involved in homeostasis and the immune response. CXCR3 is one of 20 chemokine receptors, is activated by three chemokines, CXCL9, 10, and 11, and is involved in recruiting CD4+ type-1 helper (Th1) or CD8+ cytotoxic T-cells to inflammatory tissues to initiate an appropriate immune response. These three chemokines are differentially regulated and, therefore, do not have redundant roles in vivo although they compete for binding to CXCR3 in cellular studies. These chemokines and CXCR3 are also involved in various autoimmune diseases, transplant rejection and graft versus host disease, specific infections, and cancer. Therefore, these chemokines and CXCR3 are attractive therapeutic targets for a number of diseases. One compound (AMG487) failed Phase II clinical trials for psoriasis, presumably due to its in vivo metabolism. The main goal in this application is to use structural biology of CXCR3 complexed to a small molecule antagonist to provide the foundation for the design of other antagonists with the optimum properties for a therapeutic. We are also interested in a more fundamental question: how do chemokines bind their receptors? Finding an answer to this question is beyond the scope of this proposal, but based on previous experience to purify a stable chemokine-receptor complex, we intend to explore creating a clone in P. pastoris expressing CXCR3, each of its chemokine agonists, and two other proteins that result in increased the affinity and stability between CXCR3 and each of its chemokine agonists.